The present invention relates to a thermal transfer printer for multi-color printing.
In a color printer, ink of more than one color is used and prints (printed images) made by use of ink of the respective colors are superimposed over one another to depict a multi-colored picture. Recording sheet discharge operation in such a color printer will be described with reference to FIG. 20. FIG. 20 is a diagram for explaining the recording sheet discharge operation in a conventional thermal transfer printer, and it, schematically shows the printer as viewed from front.
A cylindrical platen roller 900 is rotatably supported between frames (not shown) and adapted to be driven at its end for rotation. A guide member 902 and a recording sheet guide 903 are provided along the outer periphery of the platen roller 900 to make a recording sheet 901 wind around the platen roller 900. The guide member 902 and the recording sheet guide 903 serve also as guides for the recording sheet 901 during its feeding. The guide member 902 and the recording sheet guide 903 include auxiliary rollers 905 which are rotatably provided and pressed against the outer periphery of the platen roller 900 by guide members (not shown) to thereby apply a transport force to the recording sheet.
The guide member 902 has a sheet discharge plate 915 for changing the transport passage of the recording sheet 901 in a sheet discharge direction to and from another transport passage along the platen roller 900. The sheet discharge plate 915 is so formed as to swing around a pin 915a with the aid of a drive-transmission source (not shown). Feed rollers 906 for feed-transporting the printing sheet 901 are provided just before an insertion opening which is defined by the guide member 902 and the recording sheet guide 903, and idler rollers 907 are provided rotatably to be pressed against the respective feed rollers 906 with a load p. Discharge rollers 908 for discharging the recording sheet are provided just after a discharge opening defined by the guide member 902 and a guide plate 904, and they are associated with idler rollers 907' in substantially the same manner as the feed rollers 906. A thermal head 909 is fixed on a head arm 911 which can be swung vertically around a pin 910, so that it can be pressed against the platen roller 900 with a predetermined load.
An ink ribbon 912 includes a base film on which ink of three colors of yellow, magenta and cyanine is coated in sequence, though the three colors are not shown. The ink ribbon 912 is wound around a supply reel 913 and a take-up reel 914, and the reels are respectively fitted on a supply reel base and a take-up reel base (both not shown) which generate a predetermined rotational torque.
The operation will be now described. The platen roller 900, the feed rollers 906 and the idler rollers 907 are driven by a motive power source (not shown), and the recording sheet 901 is transported to a guide unit that is constituted by the guide member 902 and the recording sheet guide 903. Further, the recording sheet 901, under the guidance of the recording sheet guide 903, passes between the platen roller 900 and the auxiliary rollers 905, winds around the platen roller 900 and stops at a position just before it is pressed by the thermal head 909. At this time, the thermal head 909 is separated from the platen roller 900, and the ink ribbon 912 is also stopped.
Subsequently, the ink ribbon 912 is moved with the aid of a sensor (not shown) until its starting end lies in place to prepare the first color of yellow. Then, the thermal head 909 is pressed on the platen roller 900 through the ink ribbon 912 and the recording sheet 901 by means of the head arm 911, and the platen roller 900 is rotated, thereby printing the first color. At this time, since the sheet discharge plate 915 has been moved around the pin 915a in the direction indicated by an arrow in FIG. 20 by means of the driving source (not shown), the sheet discharge opening defined by the guide member 902 and the guide plate 904 is closed so that the leading end of the recording sheet 901 is introduced into the transport passage between the guide member 902 and the platen roller 900. After the printing has completed, the thermal head 909 is moved upwardly, and the platen roller 900 is rotated. With the transport force exerted from the platen roller 900 and the auxiliary rollers 905, the recording sheet 901 is transported again to the position just before it is pressed by the thermal head 909.
Thereafter, the second and third colors are likewise printed. When the printing is completed, the thermal head 909 is moved upwardly, and the platen roller 900 is rotated so that the recording sheet 901 is transported to the position just before it is pressed by the thermal head 909. Then, the thermal head 909 is pressed on the platen roller 900 through the ink ribbon 912 and the recording sheet 901, and the sheet discharge plate 915 is swung around the pin 915a by the driving source (not shown), thereby closing the transport passage between the guide member 902 and the platen roller 900. When the platen roller 900 is rotated in this state, the leading end of the recording sheet 901 is guided into the discharge opening defined by the guide member 902 and the-guide plate 904, and it is introduced between the sheet discharge rollers 908 and the idler rollers 907'. Upon the leading end of the recording sheet 901 being nipped between the sheet discharge rollers 908 and the idler rollers 907', the thermal head 909 is moved upwardly, and the recording sheet 901 is completely discharged by the transport force from the sheet discharge rollers 908 and the idler rollers 907'. Thus, the operation is completed.
Apparatus having relation to that described above are disclosed, for instance in Japanese Patent Unexamined Publications Nos. 1-317779 and 1-301275.
In the above-described conventional art, the sheet discharge plate 915 is swung and the recording sheet 901 is discharged after printing of the three colors of yellow, magenta and cyanine. Accordingly, there arises a problem that printing time per sheet is so long that the printing speed is low.
If the sheet discharge plate 915 is swung and the recording sheet 901 is introduced into the sheet discharge opening at the same time as printing of the third color, printing time per sheet is shortened, and the printing speed is increased. In this case, in the middle of printing, the leading end of the recording sheet 901 is held between the sheet discharge rollers 908 and the idler rollers 907' and is exerted with another transport force. Therefore, unless the peripheral speed of the platen roller 900 completely accords with that of the sheet discharge rollers 908, there arises a problem that transport of the recording sheet 901 is disturbed to cause irregularity in the quality of a printed image.
Consequently, either of the above-described systems for discharging the recording sheet 901 involves a problem.